In many fields which utilize a mechanism for advancing an elongate rod, there is the need for a clutch mechanism for selectively engaging and axially advancing the rod a desired distance. In particular, there is the need for a mechanism that can accurately be controlled for advancing the rod smoothly in various increments that include very small distances. One such field in which such a mechanism is needed is the field of dispensers.
There are many dispensing mechanisms for dispensing fluid materials. While many of the prior art devices are concerned with "accurate" dispensing, the amount of fluid dispensed is normally not that critical. However, in the field of painting where the artist must know the exact amount of paint being applied and the form of the applied paint (e.g. in droplets, spray or a fine line, in order to achieve the artistic expression, it is critical that the artist have accurate control over the paint being dispensed in both the rate of dispensing, the quantity dispensed, and the start and stop times of the dispensing. It is also important for the artist to have a paint applicator which "feels good" so that the artistic expression can be readily transferred from the mind of the artist through the artist's hand and accurately relayed by the painting mechanism. Finally, there is the requirement for changing paint colors or types, which often means reusing an earlier paint type.
Thus, there is a need for a paint dispenser in which the paint can be contained in a removable cartridge, and in which the paint can be accurately dispensed from the cartridge in a known amount and in a known pattern. The paint dispenser must also be capable of immediately dispensing the paint, on the one hand, and of immediately terminating the dispensing of paint, on the other hand.
Most of the solutions to dispensing material from a cartridge have utilized dispensing guns which incorporate a movable rod that is insertable into one end of the dispenser and which pushes a piston through the cartridge to expel the material out the other end. The means for advancing the rod have included a plate that is canted into engagement with the rod by a lever and a rod that has ratchet teeth and a lever which engages those teeth. These devices are disclosed in U.S. Pat. Nos. 2,530,359 to Peterson; 2,561,825 to Sherbondy; 3,141,583 to Mapel et al; 4,257,267 to Parsons; and 4,330,070 to Doubleday. Unfortunately, in those devices it is not possible to have an infinitesimal control over the rod. This is because the ratchet teeth limit the rod movement to discrete steps in those models or because there is an inability to accurately control the rod movement because in models using the canted plate as a result of that design and also as a result of the eventual ware that develops between the plate and the rod being advanced.